KBI O/U: both barrels went off

I'm selling some guns for the family of a recently-deceased friend. I test-fire everything before putting it up for sale, just so I know it is functional. Anyway, one of the pieces is a KBI (made in Russia by Baikal, I believe) O/U 12 ga., single trigger. I'm not real familiar with O/Us, but knew that some have barrel selectors. Saw no selector and figured it must just go to the second barrel after the first has discharged. Well, both went off at the same time. I do know THAT'S not supposed to happen.

Anyone know anything about these things, i.e. is this something I, with my minimal 'smithing skills, should be able to fix?

Any help would be appreciated.

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Sorry, dang cap lock key-
In general, "doubling" happens when you have worn parts- usually the sear. It is holding back the cocked striker, but under recoil, slips, and the second barrel fires a milisecond after the first.

I know that some of these depend on recoil to move firing to the second barrel. As critical as tolerances could be, and the significant results of a "whoops" when a barrel discharges when you did not pull trigger (like maybe bumping the stock hard) I would be in favor of taking this one to a smith that knows HOW to adjust the parts needed for a SAFE and reliable gun.

There are 2 types of double barrel reset for the 2nd barrel when they are selective barrel (single trigger)-recoil and mechanical. You also have non-selective single trigger, which yours is. It sounds like the sear for the second barrel is malfunctioning. You need to take it in to a smith as double barrels are frustrating to work on sometimes, and you do need special tools such as a mainspring compressor.